Semiconductor devices, such as high electron mobility transistors (HEMTs), Schottky diodes, metal semiconductor field effect transistors (MESFETS) and the like employ a Schottky contact. Schottky contacts are generally metal contacts that are formed on a semiconductor material to create a metal-semiconductor junction that tends to provide a rectifying effect due to an inherent potential barrier that is formed at the metal-semiconductor junction.
A Schottky contact may be created by forming one or more layers on the surface of a semiconductor body where the Schottky contact is desired. For example, a Schottky gate contact for a HEMT maybe formed on the semiconductor body and between corresponding source and drain contacts. If the semiconductor material on which the HEMT's Schottky gate contact is formed includes aluminum gallium nitride (AlGaN), two common metals that are often used in a Schottky gate contact for a HEMT are nickel (Ni) and gold (Au). A Ni layer may be formed over the semiconductor body, and an Au layer may be formed over the Ni layer. Notably, other layers may be provided between the Ni and Au layers.
The Ni layer is formed over the AlGaN to form the lower Schottky layer of the Schottky gate contact. Ni is often used with AlGaN due to the relatively high barrier height provided between these two materials. The upper Au layer is formed at or near the top of the Schottky gate contact to form a contact layer. Use of Au for the contact layer helps to minimize resistance associated with the Schottky gate contact.
Unfortunately, when Ni and Au are used in the different layers of a Schottky gate contact, the Ni in the Schottky layer tends to diffuse into the Au of the contact layer, and the Au of the contact layer tends to diffuse into the Ni of the Schottky layer at elevated fabrication and operational temperatures. As such, the Ni and Au in the Schottky gate contact mix with each other. These issues are not limited to Ni and Au. For example, Aluminum (Al) is often used as an alternative to Au for the contact layer, and Al readily mixes with Ni.
When the various metals in the Schottky gate contact mix with one another, the performance of the Schottky gate contact, and thus the HEMT as a whole, is often significantly degraded. For example, the leakage currents of the Schottky gate contact may significantly increase to unacceptable levels. Similar issues arise in other devices, such as Schottky diodes and MESFETs, which employ Schottky contacts, and in devices that are fabricated using other material systems.
Accordingly, there is a need for an improved Schottky contact that substantially prevents, or at least significantly reduces, the extent to which metals from the different layers mix with one another during fabrication and operation.